Building a Simple Currency Converter

This tutorial introduces the Prado web application framework and teaches you how to build a simple web application in a few simple steps. This tutorial assumes that you are familiar with PHP and you have access to a web server that is able to serve PHP5 scripts.

In this tutorial you will build a simple web application that converts a dollar amount to an other currency, given the rate of that currency relative to the dollar. The completed application is shown bellow. You can try the application locally or at the Prado website. Notice that the application still functions exactly the same if javascript is not available on the user's browser.

Downloading and Installing Prado

To install Prado, simply download the latest version of Prado from the Prado project website and unzip the file to a directory not accessible by your web server (you may unzip it to a directory accessible by the web server if you wish to see the demos and test). For further detailed installation, see the Quickstart Installation guide.

Creating a new Prado web Application

The quickest and simplest way to create a new Prado web application is to use the command tool prado-cli.php found in the framework directory of the Prado distribution. We create a new application by running the following command in your command prompt or console. The command creates a new directory named currency-converter in your current working directory. You may need to change to the appropriate directory first. See the Command Line Tool for more details.

php prado/framework/prado-cli.php -c currency-converter

The above command creates the necessary directory structure and minimal files (including "index.php" and "Home.page") to run a Prado web application. Now you can point your browser's url to the web server to serve up the index.php script in the currency-converter directory. You should see the message "Welcome to Prado!"

Creating the Currency Converter User Interface

We start by editing the Home.page file found in the currency-converter/protected/pages/ directory. Files ending with ".page" are page templates that contains HTML and Prado controls. We simply add two textboxes, three labels and one button as follows.

<com:TForm>
    <fieldset>
        <legend>Currency Converter</legend>
        <div class="rate-field">
            <com:TLabel ForControl="currencyRate" Text="Exchange Rate per $1:" />
            <com:TTextBox ID="currencyRate" />
        </div>
        <div class="dollar-field">
            <com:TLabel ForControl="dollars" Text="Dollars to Convert:" />
            <com:TTextBox ID="dollars" />
        </div>
        <div class="total-field">
            <span class="total-label">Amount in Other Currency:</span>
            <com:TLabel ID="total" CssClass="result" />
        </div>
        <div class="convert-button">
            <com:TButton Text="Convert" />
        </div>
    </fieldset>
</com:TForm>

If you refresh the page, you should see something similar to the following figure. It may not look very pretty or orderly, but we shall change that later using CSS.

The first component we add is a TForm that basically corresponds to the HTML <form> element. In Prado, only one TForm element is allowed per page.

The next two pair of component we add is the TLabel and TTextBox that basically defines a label and a textbox for the user of the application to enter the currency exchange rate. The ForControl property value determines which component that the label is for. This allows the user of the application to click on the label to focus on the field (a good thing). You could have used a plain HTML <label> element to do the same thing, but you would have to find the correct ID of the textbox (or <input> in HTML) as Prado components may/will render the ID value differently in the HTML output.

The next pair of components are similar and defines the textbox to hold the dollar value to be converted. The TLabel with ID value "total" defines a simple label. Notice that the ForControl property is absent. This means that this label is simply a simple label which we are going to use to display the converted total amount.

The final component is a TButton that the user will click to calculate the results. The Text property sets the button label.

Implementing Currency Conversion

If you tried clicking on the "Convert" button then the page will refresh and does not do anything else. For the button to do some work, we need to add a "Home.php" to where "Home.page" is. The Home class should extends the TPage, the default base class for all Prado pages.

<?php
class Home extends TPage
{

}

Prado uses PHP's __autoload method to load classes. The convention is to use the class name with ".php" extension as filename.

So far there is nothing interesting about Prado, we just declared some "web components" in some template file named Home.page and created a "Home.php" file with a Home class. The more interesting bits are in Prado's event-driven architecture as we shall see next.

We want that when the user click on the "Convert" button, we take the values in the textbox, do some calculation and present the user with the converted total. To handle the user clicking of the "Convert" button we simply add an OnClick property to the "Convert" button in the "Home.page" template and add a corresponding event handler method in the "Home.php".

<com:TButton Text="Convert" OnClick="convert_clicked" />

The value of the OnClick, "convert_clicked", will be the method name in the "Home.php" that will called when the user clicks on the "Convert" button.

class Home extends TPage
{
    public function convert_clicked($sender, $param)
    {
        $rate = floatval($this->currencyRate->Text);
        $dollars = floatval($this->dollars->Text);
        $this->total->Text = $rate * $dollars;
    }
}

If you run the application in your web browser, enter some values and click the "Convert" button then you should see that calculated value displayed next to the "Amount in Other Currency" label.

In the "convert_clicked" method the first parameter, $sender, corresponds to the object that raised the event, in this case, the "Convert" button. The second parameter, $param contains any additional data that the $sender object may wish to have added.

We shall now examine, the three lines that implements the simply currency conversion in the "convert_clicked" method.

$rate = floatval($this->currencyRate->Text);

The statement $this->currencyRate corresponds to the TTextBox component with ID value "currencyRate" in the "Home.page" template. The Text property of the TTextBox contains the value that the user entered. So, we obtain this value by $this->currencyRate->Text which we convert the value to a float value.

$dollars = floatval($this->dollars->Text);

The next line does a similar things, it takes the user value from the TTextBox with ID value "dollars and converts it to a float value.

The third line calculates the new amount and set this value in the Text property of the TLabel with ID="total". Thus, we display the new amount to the user in the label.

$this->total->Text = $rate * $dollars;

Adding Validation

The way we convert the user entered value to float ensures that the total amount is always a number. So the user is free to enter what ever they like, they could even enter letters. The user's experience in using the application can be improved by adding validators to inform the user of the allowed values in the currency rate and the amount to be calcuated.

For the currency rate, we should ensure that

  1. the user enters a value,
  2. the currency rate is a valid number,
  3. the currency rate is positive.

To ensure 1 we add one TRequiredFieldValidator. To ensure 2 and 3, we add one TCompareValidator. We may add these validators any where within the "Home.page" template. Further details regarding these validator and other validators can be found in the Validation Controls page.

<com:TRequiredFieldValidator
	ControlToValidate="currencyRate"
	ErrorMessage="Please enter a currency rate." />
<com:TCompareValidator
	ControlToValidate="currencyRate"
	DataType="Float"
	ValueToCompare="0"
	Operator="GreaterThan"
	ErrorMessage="Please enter a positive currency rate." />

For the amount to be calculated, we should ensure that

  1. the user enters a value,
  2. the value is a valid number (not including any currency or dollar signs).

To ensure 1 we just add another TRequiredFieldValidator, for 2 we could use a TDataTypeValidator. For simplicity we only allow the user to enter a number for the amount they wish to convert.

<com:TRequiredFieldValidator
	ControlToValidate="dollars"
	ErrorMessage="Please enter the amount you wish to calculate." />
<com:TDataTypeValidator
	ControlToValidate="dollars"
	DataType="Float"
	ErrorMessage="Please enter a number." />

Now if you try to enter some invalid data in the application or left out any of the fields the validators will be activated and present the user with error messages. Notice that the error messages are presented without reloading the page. Prado's validators by default validates using both javascript and server side. The server side validation is always performed. For the server side, we should skip the calculation if the validators are not satisfied. This can done as follows.

public function convert_clicked($sender, $param)
{
    if($this->Page->IsValid)
    {
        $rate = floatval($this->currencyRate->Text);
        $dollars = floatval($this->dollars->Text);
        $this->total->Text = $rate * $dollars;
    }
}

Improve User Experience With Active Controls

Requires Prado versions 3.1a or later.

In this simple application we may further improve the user experience by increasing the responsiveness of the application. One way to achieve a faster response is calculate and present the results without reloading the whole page.

We can replace the TButton with the Active Control counter part, TActiveButton, that can trigger a server side click event without reloading the page. In addition, we can change the "totals" TLabel with the Active Control counter part, TActiveLabel, such that the server side can update the browser without reloading the page.

<div class="total-field">
    <span class="total-label">Amount in Other Currency:</span>
         <com:TActiveLabel ID="total" CssClass="result" />
    </div>
    <div class="convert-button">
        <com:TActiveButton Text="Convert" OnClick="convert_clicked" />
</div>

The server side logic remains the same, we just need to import the Active Controls name space as they are not included by default. We add the following line to the begin of "Home.php".

Prado::using('System.Web.UI.ActiveControls.*');

If you try the application now, you may notice that the page no longer needs to reload to calculate and display the converted total amount. However, since there is not page reload, there is no indication or not obvious that by clicking on the "Convert" button any has happened. We can further refine the user experience by change the text of "total" label to "calculating..." when the user clicks on the "Convert" button. The text of the "total" label will still be updated with the new calculate amount as before.

To indicate that the calculation is in progress, we can change the text of the "total" label as follows. We add a ClientSide.OnLoading property to the "Convert" button (since this button is responsible for requesting the calculation).

<com:TActiveButton Text="Convert" OnClick="convert_clicked" >
    <prop:ClientSide.OnLoading>
        $('<%= $this->total->ClientID %>').innerHTML = "calculating..."
    </prop:ClientSide.OnLoading>
</com:TActiveButton>

The ClientSide.OnLoading and various other properties accept a javascript block as their content or value. The javascript code $('...') is a javascript function that is equivalent to document.getElementById('...') that takes a string with the ID of an HTML element. Since Prado renders its components's IDs, we need to use the rendered ID of the "total" label, that is, $this->total->ClientID. We place this bit of code within a <%= %> to obtain the rendered HTML ID for the "total" label. The rest of the javascript code innerHTML = "calculating..." simply changes the content of the "total" label.

Adding Final Touches

So far we have built a simple currency converter web application with little attention of the looks and feel. Now we can add a stylesheet to improve the overall appearance of the application. We can simply add the stylesheet inline with the template code or we may create a "theme".

To create and use a theme with Prado applications, we simply create a new directory "themes/Basic" in the currency-converter directory. You may need to create the themes directory first. Any directory within the themes are considered as a theme with the name of the theme being the directory name. See the Themes and Skins for further details.

We simply create a CSS file named "common.css" and save it in the themes/Basic directory. Then we add the following code to the beginning of "Home.page" (we add a little more HTML as well).

<%@ Theme="Basic" %>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8" />
</head>
<body>

The first line <%@ Theme="Basic" %> defines the theme to be used for this page. The THead corresponds to the HTML <head> element. In addition to display the Title property by the THead, all CSS files in the themes/Basic directory are also rendered/linked for the current page. Our final currency converter web application looks like the following. This completes introduction tutorial to the Prado web application framework.